feat: gradient buttons, language selector with flags, SVG support, and UI improvements

- Add linear gradient backgrounds to primary/secondary/danger buttons
- Add language selector with flag icons in ComboBox
- Add SVG support via egui_extras feature
- Add cancel confirmation dialog on exit
- Update Yali logo to SVG icon
- Add flag SVGs for all countries
- Improve users step layout with form helpers
- Refactor button API (ui::theme::*_button now takes ui param)

assets/flags/dj.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-dj" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="dj-a">
+      <path fill-opacity=".7" d="M-40 0h682.7v512H-40z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#dj-a)" transform="translate(37.5)scale(.94)">
+    <path fill="#0c0" d="M-40 0h768v512H-40z"/>
+    <path fill="#69f" d="M-40 0h768v256H-40z"/>
+    <path fill="#fffefe" d="m-40 0 382.7 255.7L-40 511z"/>
+    <path fill="red" d="M119.8 292 89 270l-30.7 22.4L69.7 256l-30.6-22.5 37.9-.3 11.7-36.3 12 36.2h37.9l-30.5 22.7 11.7 36.4z"/>
+  </g>
+</svg>